Generally, in order to control a permanent magnet motor, characteristics of the permanent magnet motor such as the phase, the frequency and the amplitude of the electric motive force (“emf”) voltage generated by the rotation of the motor rotor, need to be determined to yield a voltage to be applied to the motor terminals.
These characteristics of the permanent magnet motor may be obtained by using a position sensor, which results in increased costs and reduces the reliability of the method because feedback signals are subject to changes in the ambient environment such as noise and temperature and to the presence of impurity for example.
A possible method involves estimating the emf of the permanent magnet motor. However, in case of a high-speed motor this method requires a high computation speed, which may result costly. Moreover, since the characteristics of the motor are dependent on the ambient environment, such a control method can be complex.
European Patent EP0944164 teaches a system and a method for controlling a permanent magnet electric motor using an induced voltage estimator.
From the foregoing, it appears that although a number of methods are known to control permanent magnet motors, these methods either require position sensors and complicated computation or must be adapted to the environment according to each design of permanent magnet motors.
Therefore, there is a need for a system and a method, which allow controlling a permanent magnet electric motor in a simple, reliable way and which automatically adapts to environmental changes.